1. Field of the Invention
The invention relates to sensor circuitry and, in particular to circuitry for driving a sensor transducer and processing the sensor output to produce a usable output signal.
2. Description of the Prior Art
The sensor output may have inaccuracies therein as a result of thermal sensitivity of the sensor or because of residual signals which may occur in a sensor, such as hysteresis, residual magnetism, etc., depending upon the type of sensor.
Also, many sensor devices produce very low level output signals. In order that these signals can be utilized to drive associated devices, such as indicators and the like, the signal must first be amplified. The amplifier circuitry may introduce errors which are commonly referred to as "offset". One type of amplifier offset is that introduced by thermal drift. The amplifier circuitry is temperature-sensitive, so that the output may vary with temperature, independently of the input signal. Another type of amplifier offset which occurs in operational amplifiers is any voltage other than zero which appears at the output of the device when the input terminals are connected together, i.e., when there is no input signal. Such an offset may result, for example from a mismatch of the active elements of the device or of the load impedances.
It is known to correct such offsets by various error correction techniques. Such techniques include offset nulling or zeroing techniques wherein a correction signal is algebraically added to the amplified output signal to cancel the offset error therein. Sample and hold circuits have been utilized in such offset nulling arrangements. Such circuits typically attempt to derive from the output signal a signal which is proportional to the offset component thereof and then algebraically sum that derived signal with the output signal to produce a corrected output. But these prior arrangements have been used with a continuous analog output signal from the sensor. Thus, complex circuitry has been utilized to produce a correction signal which is proportional only to the offset.
Furthermore, many of these prior devices are relatively high-power arrangements, wherein the sensor circuitry typically draws a current of 25-30 ma. Such circuitry is not suitable for battery-operated applications, either the battery life is too short, thus requiring either frequent battery replacement or the use of expensive and inconvenient rechargeable batteries.
In applications other than sensor circuitry, it has been known to reduce the power consumption of battery-operated devices by providing an intermittent operation, wherein the circuitry draws power only during very short periodically occurring intervals. If the duty cycle, i.e., the ratio of the duration of the ON interval to the duration of the time between the beginning of successive ON intervals, is very small, substantial power savings can be realized.